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Model 60B Tapped Power Resistor #7998 - Printable Version

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Model 60B Tapped Power Resistor #7998 - Crist Rigotti - 04-11-2015

I need a little help.  I'm getting ready to restore a late run Model 60 and I'm going over all the parts and their locations to familiarize myself with the set before I dig in.  I plan on replacing all the capacitors and resistors, and of course repairing both coils.  My question concerns part #47 on the schematic Philco p/n 7998.  It is a tapped power resistor.  When doing a very quick look over I homed it and it checked good.  It doesn't look that good but it did check out OK.  The schematic shows that it is a 32 ohm and a 235 ohm resistor.  On the Philco websites this resistor is not one of those listed.  

What I'm trying to do is to find out what the wattage would be. It looks like at least a 10 watts.   I'd like to know this just in case I need to replace it with a modern resistor.  I can replace components and spiff up the chassis etc but I'm lacking the knowledge to look at the schematic and figure it out by myself.  I have the spec on the #80 rectifier tube but have little idea what to do with them to solve my question.

Would somebody please tell me how to figure it out.  I'm familiar with Ohms Law and I know what the resistance is but I'm unsure of what the voltage and current would be across this resistor.

I'm attaching the schematic for the radio.  It is #47 on the schematic.

Thanks in advance.  

(Ron, I know this isn't your favorite radio but I got it pretty cheap and I thought it would be good to do for the experience before I do the 116X.)

RE: Model 60B Tapped Power Resistor #7998 - morzh - 04-11-2015

You could simply add tubes' currents, would be enough for calculation.
Looks like 45mA is fairly conservative, so total dissip is 0.5W.
So 2W resistor would be enough. This goes for 235 Ohms one as it dissipates 90%. The 32 ohm can be under 1W.

PS. As long as the resistor checks OK, no matter how it looks, clean it and leave it be.

RE: Model 60B Tapped Power Resistor #7998 - Radioroslyn - 04-11-2015

Those bias divider resistors can and sometimes look pretty nasty from heating. But as long as it checks ok resistance wise  your good to go. Having leaky and or shorted caps don't help it's appearance!
GL
Terry

RE: Model 60B Tapped Power Resistor #7998 - Crist Rigotti - 04-12-2015

(04-11-2015, 09:12 PM)morzh Wrote:  You could simply add tubes' currents, would be enough for calculation.
Looks like 45mA is fairly conservative, so total dissip is 0.5W.
So 2W resistor would be enough. This goes for 235 Ohms one as it dissipates 90%. The 32 ohm can be under 1W.

PS. As long as the resistor checks OK, no matter how it looks, clean it and leave it be.

Wow, that was simple.  A lot less wattage than I thought just by looking at the one that is in there now.  I'm going to clean it then leave it alone, but I'll get a couple of resistors just in case.  

Thank you.

RE: Model 60B Tapped Power Resistor #7998 - Crist Rigotti - 04-12-2015

(04-11-2015, 11:00 PM)Radioroslyn Wrote:  Those bias divider resistors can and sometimes look pretty nasty from heating. But as long as it checks ok resistance wise  your good to go. Having leaky and or shorted caps don't help it's appearance!
GL
Terry

Terry,
You're right!  I looked at it a little closer and it have cloth covered wire wrapped around it!  I'll clean it and be careful how I handle it.  Appreciate the reply.

RE: Model 60B Tapped Power Resistor #7998 - Radioroslyn - 04-12-2015

Rather than trying to clean it I might try using a heat gun to reflow whats already there.
Terry

RE: Model 60B Tapped Power Resistor #7998 - Crist Rigotti - 04-12-2015

I'll keep that in mind Terry.

I took a few pics of the resistor.  One of the things that had me concerned was the wire sticking out near the terminal closest to the chassis.  I used a magnifier and a bright light and it looks like excess lead after wrapping around the terminal.  Also under the bright light, it doesn't look that bad after all considering the radio is nearly 80 years old.

Yeah, when getting familiar with the chassis and the schematic I use small pieces of masking tape to ID the components.  It seems to be the only way for me to keep track of what's what!  Aids me in tracing out the schematic too.

RE: Model 60B Tapped Power Resistor #7998 - Radioroslyn - 04-12-2015

Wow that's good looking one! You should see a BAD one. Got alot of loose wires there.


Terry

RE: Model 60B Tapped Power Resistor #7998 - Crist Rigotti - 04-12-2015

(04-12-2015, 09:16 AM)Radioroslyn Wrote:  Wow that's good looking one! You should see a BAD one. Got alot of loose wires there.


Terry

I'm looking forward to getting started on it in earnest in a few weeks. 

Yeah, the chassis did not have a transformer so I got another one.  Whoever took the transformer just cut the wires and took it.  When I'm done with it, it'll look good again.